The present invention relates generally to systems and methods for applying electrical energy to a patient and more specifically to steerable electrophysiology catheters for use in mapping and/or ablation of the heart.
The heart is primarily composed of multiple fibers which are responsible for the propagation of signals necessary for normal, electrical and mechanical function. The presence of an arrhythmogenic site or abnormal pathway which may bypass or short circuit the normal conducting fibers in the heart often causes abnormally rapid rhythms of the heart, which are referred to as tachycardias. Tachycardias may be defined as ventricular tachycardias (VTs) and supraventricular tachycardias (SVTs). VTs originate in the left or right ventricle and are typically caused by arrhythmogenic sites associated with ventricular myocardial disease. SVTs originate in the atria or the atrioventricular (AV) junction and are frequently caused by abnormal circuits or foci.
The present invention is concerned with the treatment of atrial fibrillation and atrial flutter, which are two of the most common sustained cardiac arrhythmias and of the major causes of systemic embolism. Therapy for patients suffering from atrial fibrillation usually focuses on controlling the symptoms (palpitations, angina, dyspnea, syncope and the like), improving cardiac performance and reducing the risk of thromboembolism. Treatment of atrial fibrillation may be accomplished by a variety of approaches, including drugs, surgery, implantable pacemakers/defibrillators, and catheter ablation. While antiarrhythmic drugs may be the treatment of choice for many patients, these drugs may only mask the symptoms and do not cure the underlying cause. Implantable devices, on the other hand, usually can correct an arrhythmia only after it occurs. Surgical and catheter-based treatments, by contrast, may actually cure the problem usually by ablating the abnormal arrhythmogenic tissue or abnormal pathway responsible for the atrial fibrillation or flutter. The catheter-based treatments rely on the application of various destructive energy sources to the target tissue including direct current electrical energy, radiofrequency electrical energy, microwave energy laser energy, cryoenergy, ultrasound and the like.
Of particular interest to the present invention are radiofrequency (RF) ablation protocols which have proven to be effective in treatment of atrial fibrillation while exposing the patient to minimum side effects and risks. Radiofrequency catheter ablation may be performed after an initial mapping procedure where the locations of the arrhythmogenic sites and abnormal pathways are determined. A catheter having a suitable electrode is introduced to the appropriate heart chamber and manipulated so that the electrode lies proximate the target tissue. Radiofrequency energy is then applied through the electrode to the cardiac tissue to ablate a region of the tissue which forms part of the arrhythmogenic site or the abnormal pathway. By successfully destroying that tissue, the abnormal conducting patterns responsible for the atrial fibrillation or flutter cannot be sustained. Methods and systems for performing RF ablation by controlling temperature at the ablation site are described in co-pending application Ser. No. 07/866,683, now U.S. Pat. No. 5,573,533 entitled "Method and System for Radiofrequency Ablation of Cardiac Tissue," filed Apr. 10, 1992, the complete disclosure of which is hereby incorporated by reference.
Catheters designed for mapping and/or ablation frequently include a number of individual electrode bands mounted to the distal tip of the catheter so as to facilitate mapping of a wider area in less time, or to improve access to target sites for ablation. Such catheters are described in co-pending application Ser. No. 07/866,383, now U.S. Pat. No. 5,318,525 filed Apr. 10, 1992, the complete disclosure of which is hereby incorporated by reference. Catheters utilized in radiofrequency ablation are typically inserted into a major vein or artery, usually in the neck or groin area, and guided into the chambers of the heart by appropriate manipulation through the vein or artery. Such catheters must facilitate manipulation of the distal tip or ablation segment so that the distal electrode(s) can be positioned against the tissue region to be ablated. The catheter must have a great deal of flexibility to follow the pathway of the major blood vessels into the heart, and the catheter must permit user manipulation of the distal ablation segment even when the catheter is in a curved and twisted configuration. Because of the high degree of precision required for proper positioning of the tip electrode, the catheter must allow manipulation with a high degree of sensitivity and controllability.
An important factor which has driven the recent development of curative catheter ablation therapies for atrial fibrillation has been the development of a successful surgical procedure, the "Maze" procedure, for treating patients with this arrhythmia. The Maze procedure was developed to provide both sinus node control of ventricular rate and effective, appropriately synchronized biatrial contraction. This procedure involves opening the patient's chest cavity with a gross thoracotomy, usually in the form of a median sternotomy, to gain access into the patient's thoracic cavity, and cutting long linear incisions through the heart wall to electrically partition portions of the heart. In particular, the Maze procedure partitions the atria such that: (1) no portion of the atrium is large enough to support atrial fibrillation; (2) conduction of the sinus impulse to the AV node and to most portions of the atria is maintained; and (3) relatively normal atrial contraction is restored.
The success of the Maze procedure has driven interest in the development of a catheter ablation procedure which can replicate the therapeutic results of the surgical Maze procedure. This catheter ablation procedure involves the creation of relatively long linear lesions along the heart tissue with the distal tip of an ablation catheter. This desire to produce linear lesions has led to catheter designs in which several ablation electrodes are mounted on the length of the distal ablation segment of the catheter shaft. By engaging the entire length of the distal ablation segment with the heart tissue, a string of "point" lesions are connected together to form a linear lesion. When contact of the distal ablation segment shaft is attempted with standard catheter designs, however, the line of force to the ablation segment is typically applied from the proximal end, while the distal tip of the ablation segment remains relatively free. Consequently, contact pressure of the distal tip of such a catheter is dependent upon the relative stiffness of the distal ablation segment, and the translation of force by the operator through the main catheter shaft to the proximal portion of the ablation segment. This inefficient application of force along the distal ablation segment of the catheter results in non-uniform (and often inadequate) application of contact pressure at each point along the distal ablation segment.